1. Technical Field of the Invention
The present invention relates to an imaging lens that forms an image of an object on an image receiving surface.
2. Description of the Related Art
Recently, a photographic camera module has come to be provided to portable terminals such as cellular phones. This type of equipment needs miniaturizing of the whole camera module including an optical system, an imaging element, and an image processing device in order to enhance the portability thereof. Regarding imaging elements such as a CCD and a CMOS, a high resolution and small imaging element with miniaturized pixel size has been rapidly spreading. As an optical system responding to such an imaging element, an optical system that satisfies all needs such as small size, low price, high resolution and excellent optical performance is required.
In such an optical system, specific requirements are roughly categorized as below.                low price (the number of lenses is small, resin is available, easy to be formed, and easy to be assembled)        bright (small Fno)        small (specifically, the length from the end of a lens to an imaging element is short)        large angle of view        uniform field illumination (vignetting is small, and small incident angle of a light beam to an imaging element)        high resolution (basic aberrations such as a spherical aberration, a coma, a curvature of field, astigmatism, and an chromatic aberration are excellently corrected)        excellent depicting ability (distortion is free, flare is small, and the like)        
If an optical system that satisfies all of the above requirements is realized with a small number of lenses, the scope of application thereof greatly spreads. Meanwhile, since the recording density of an imaging element used in a portable terminal and the like has been rapidly increased recently, the situation has come where a lens system formed of one or two lenses that were used in an imaging element initial stage can not obtain sufficient versatility of design and can not satisfy a required specification.
Regarding a two-lens structure, a structure of negative-positive in this order from an object is favorable in view of aberration correction but has a limit of being miniaturized. In the two-lens structure, in order to shorten the total length thereof, a structure of positive-positive or positive-negative is preferable. In addition, in order to keep an incident angle of a light beam to an imaging element small, a structure of “front diaphragm” where an aperture stop is disposed closest to an object in the system is preferable. As examples of the two-lens structure that satisfy such structural requirements among examples that have been proposed, there are examples disclosed in Japanese laid-open publication No. 1-245211 and Japanese laid-open publication No. 4-211214, and an example proposed by the present inventor where a positive lens and a meniscus lens with a concave surface facing an object side are combined.
FIG. 16 is a sectional view of the lens system disclosed in Japanese laid-open Publication No. 1-245211. The lens system is formed of a biconvex positive first lens and a negative meniscus second lens with a concave surface facing an image side. In this example, since the power of the second lens is relatively strong and a surface on the image side thereof is a concave surface, an incident angle of a light beam to an imaging element tends to be large. Therefore, increasing an angle of view is difficult, so that the angle of view in the embodiment also remains at about 20 degrees, which is small.
FIG. 17 is a sectional view of the lens system disclosed in Japanese laid-open publication No. 4-211214. The lens system is formed of an imaging lens 10 disposed on an object side and a correction lens 20 disposed on an image side of the imaging lens 10. In this example, the imaging lens 10 provides almost all power of the whole system and the correction lens 20 disposed on the image side is a biaspheric lens, thereby correcting field aberrations and balancing each aberration when the angle of view is increased. In this structure, since the imaging lens 10 on the object side takes charge of basic imaging operation, if the lens on the object side is formed of a single lens, there is a limit to balance field aberrations only by the correction lens 20 with the incident angle of a light beam to an imaging element being decreased. In addition, since effective correction of chromatic aberration is difficult, the widening of the angle of view is limited.
FIG. 18 shows a combination of a positive lens and a meniscus lens with a concave surface facing an object side that has been proposed by the present inventor. The aberrations of the lens system are balanced and miniaturizing thereof is possible, considering it is a two-lenses. However, the versatility of designing thereof is insufficient to respond to a further increase in resolution of an imaging element.
The present invention is intended to provide a bright, small, and inexpensive imaging lens system with short total length whose aberrations are excellently corrected and that matches an angle of view of over 30 degrees and responds to a need for high resolution.